Method for reducing surface particle shedding

ABSTRACT

A method for reducing surface particle shedding of a material, specifically a suspension mounting arm for a hard disk drive, made of an alloy such as 300 Series stainless steel, by thermally conditioning the mounting arm for a desirable time period sufficient to reduce residual stress in the mounting arm while substantially maintaining its original yield strength characteristics. To ensure fabrication free from oxidation and contamination, the contained heat source can be oxygen depleted by evacuating the interior of the container to a sub-atmospheric pressure or by filling the interior of with an inert gas such as argon or a reducing gas, such as hydrogen. After thermal exposure, the material is cooled to an ambient temperature. The resulting material exhibits adequate stiffness and yield strength for assembly and operation in a hard disk drive.

FIELD OF THE INVENTION

The invention relates generally to a method for treating stainless steel components to reduce surface particle shedding while maintaining functional stiffness. More particularly, the invention relates to a method for treating stainless steel used for hard disk drive suspension mounting arms.

DESCRIPTION OF THE RELATED ART

A key component of any computer system is a device to store data. One common place for storing massive amounts of data in a computer system is on a hard disk drive (HDD). The most basic parts of a disc drive are a disc that is rotated, an actuator structure that moves a transducer to various locations on the disc, and electrical circuitry that is used to write and read data to and from the disc. There are a variety of disc drives in use today, such as hard disc drives, zip drives, floppy disc drives. All utilize either rotary or linear actuators.

In hard disk drives, magnetic heads read and write data on the surfaces of rotating disks that are co-axially mounted on a spindle motor. The magnetically-written “bits” of information are laid out in concentric circular “tracks” on the surfaces of the disks. The disks must rotate quickly so that the computer user does not have to wait long for a desired bit of information on the disk surface to translate to a position under the head. In modern disk drives, data bits and tracks must be extremely narrow and closely spaced to achieve a high density of information per unit area of the disk surface.

The required small size and close spacing of information bits on the disk surface have consequences on the design of the disk drive device and its mechanical components. Among the most important consequences is that the magnetic transducer on the head must operate in extremely close proximity to the magnetic surface of the disk. Because there is relative motion between the disk surface and the magnetic head due to the disk rotation and head actuation, continuous contact between the head and disk can lead to tribological failure of the interface. Such tribological failure, known colloquially as a “head crash,” can damage the disk and head and cause data loss. Therefore, the magnetic head is designed to be hydrodynamically supported by an extremely thin air bearing so that its magnetic transducer can operate in close proximity to the disk while physical contact between the head and the disk is minimized or avoided. Typically, the head-to-disk spacing present during operation of modern hard disk drives is extremely small, measuring in the tens of nanometers.

Characteristics of the actuator used for moving the magnetic transducer in close proximity to the disk, must be considered by the designer to minimize vibration in response to rapid angular motions and other excitations. For example, the mounting arm must be stiff enough and the mounting arm pivot bearing must be of high enough quality so that the position of the head can be precisely controlled during operation. Also, the interface between the mounting arm and the pivot bearing must be of sufficient rigidity and strength to enable precise control of the head position during operation and to provide the boundary conditions necessary to facilitate higher natural resonant frequencies of vibration of the mounting arm. The stiffness of the mounting arm must also be sufficient to limit deflection that might cause contact with the disk during mechanical shock events.

Despite these design considerations, tribological failures may still occur due to mounting arm resonance from air fluid turbulence, sudden shocks from being dropped, and debris and contamination from the environment. Resonance and sudden shocks can facilitate surface particle shedding of the mounting arm material. The particle residue-can force the magnetic head away from the disk. The resulting temporary increase in head-disk spacing can cause read/write errors-limit the activation speeds and achievable tracks per inch (TPI) on the disk, and cause data loss. The particle residue can also get lodged between the read/write head and the disk. This creates defects on the disk that may result in a head crash.

Most state-of-the-art attempts for improving post-fabrication cleanliness of disk drive components have focused on pre- and post-assembly cleaning steps and on environmental cleanliness during assembly. The fallacy of this approach is that the material continues to shed particles aided by the build up of stress near the surface. The industry's marked reliance on post-assembly cleaning steps has increased even though such steps are impractical in high volume production. Assembly in clean environments does not prevent the generation of contaminants and debris during assembly operations performed within those clean environments; nor does it reduce surface particle shedding of the mounting arm structure during operation. Less frequently, disk drive designers consider surface particle shedding of mounting arms earlier in the design of sub-components. Consequently, there remains a need in the art for reducing surface particle shedding of the mounting arm material itself.

The method of the present invention significantly reduces the surface particle shedding of stainless steel mounting arms without substantially affecting its structural and functional characteristics. Particle shedding of stainless steel mounting arms occur more frequently because they are designed to be high hardness structures. Prior art attempts have focused on pre- and post-assembly cleaning, but have not addressed this cause for increased shedding.

Therefore, there is a need in the art for a mounting arm that can generally reduce surface particle shedding in addition to debris removal by post-assembly cleaning steps. Although the need in the art was described above in the context of an arm for magnetic disk drive information storage devices, the need is also present in other applications where steel is used in a clean environment that must remain as free as possible of particulate residue.

SUMMARY OF THE INVENTION

The invention is directed to a method for thermally conditioning stainless steel mounting arms so that the material is softened, thereby reducing or relieving the residual stress that aids in surface particle shedding. The method of the invention thermally conditions a mounting arm in an oven or furnace at a temperature and duration sufficient to anneal the outer surface of the mounting arm. The temperature is maintained between approximately 600° F. and 1850° F. The oven environment can be oxygen depleted and/or filled with an inert or reducing gas. The oven environment can also be evacuated to a sub-atmospheric pressure. The heated mounting arm is removed from the oven and air cooled to ambient temperature. The resulting grain structure of the mounting arm is softened only to the point of reducing or relieving stress, while the overall yield strength of the mounting arm is maintained adequate for its functionality. Another method according to the invention thermally conditions a mounting arm in an oven or furnace at a temperature sufficient to anneal the entire mounting arm.

BRIEF DESCRIPTION OF THE DRAWINGS

Many of the advantages, object and features of the invention will become readily appreciated by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference numerals description like parts throughout the figures, and wherein:

FIG. 1 is a perspective view of a mounting arm that can be thermally conditioned by a method embodying the present invention.

FIG. 2 is a schematic diagram showing the mounting arm of FIG. 1 inside an oven for processing in accordance with a method of the present invention.

FIG. 3 is an exemplary flow chart depicting a method for reducing surface particle shedding of a mounting arm according to one embodiment of the present invention.

FIG. 4 is a graph of particles per part vs. heating temperature, illustrating the effect of temperature treatment on surface particle shedding of a mounting arm in accordance to a method embodying the present invention.

FIG. 5 is a graphical representation illustrating a relative decrease in surface particle shedding of a mounting arm treated in accordance with a method of the present invention.

DETAILED DESCRIPTION

FIG. 1 illustrates a perspective view of a typical mounting arm 100 that may be thermally conditioned by a method embodying the present invention. The mounting arm 100 can be fabricated of any alloy metal that is intended for use in an environment free of shed surface residue. The mounting arm 100 may be made from 300 Series stainless steel material, for example. The mounting arm 100 may be made from Grade 304 Stainless Steel, which is composed of (0-0.8%)Fe, (17.5-20%)Cr, (8-11%)Ni, (0-2%)Mn, (0-1%)Si, (0-0.045%)P, and (0-0.03%)S. The mounting arm 100 is typically used to move a magnetic transducer in close proximity to a disc. It must maintain a sufficiently high stiffness to withstand high frequency resonances or mechanical shock occurring during operation of a disk drive.

The mounting arm 100 can have any shape or geometry that exhibits desirable functional and structural characteristics. For example, the mounting arm 100 can be a mounting arm typically used in hard disk drives with an opening 110 for reducing the rotational inertia of the mounting arm 100. The mounting arm 100 is typically thin, for example, around 0.012 inches.

Referring to FIGS. 2 and 3, an application for the method embodying the present invention, is generally designated 200. The mounting arm 100 is thermally conditioned using a furnace or an oven 210. Although a method utilizing an oven 210 is described herein, the method of the present invention applies equally to other heating sources such as an infrared beam or a high energy-laser. These alternative heating methods can selectively temper the surface of the mounting arm by about 20 Vickers hardness or more.

Typically, the oven 210 is heated, at step 300, to an elevated temperature sufficient to soften the mounting arm 100. For example, the temperature of the oven or furnace 210 can be maintained between approximately 600° F. and approximately 1850° F. A thermal controller 220 can be used to set the temperature of the oven 210. To ensure fabrication free from oxidation and contamination, the oven 210 can be oxygen depleted by evacuating the interior 230 to a sub-atmospheric pressure sufficient to cause a vacuum, or by filling the oven interior 230 with an inert gas such as argon or a reducing gas, such as hydrogen.

Once the oven 210 reaches the desired temperature, the mounting arm 100 is inserted, at step 310, for a desirable period. The desirable period is a function of the thickness of the mounting arm 100, the temperature at which the mounting arm 100 is exposed, and the resulting hardness desired in the stainless steel material. In one embodiment, the effect of annealing extends only partially through the mounting arm 100. For example, the desired softness is only in a thin outer surface layer of the mounting arm 100. In another embodiment, the desired softness extends throughout the mounting arm 100. Although the grain structure of the material is softened, stiffness of the mounting arm 100 is maintained. Thus, the mounting arm 100—has less stress at least in the surface layer.

After thermal conditioning, the mounting arm 100 is removed, at step 320, from the oven, and allowed to cool, at step 330, to ambient temperature.

FIGS. 4 and 5 illustrate the relative decrease in surface shedding of the mounting arm 100 after treatment according to a method embodying the present invention. To assess the susceptibility to surface particle shedding the mounting arm 100 can be subjected to various particle extraction techniques. The resulting shed particle residue can then be analyzed using different procedures.

One procedure, referred to as Tape Particle Analysis (TPA), applies an adhesive material, such as an acetate tape, to the surface of the mounting arm 100. The tape adheres to the mounting arm surface and when removed, loose surface particles remain embedded in the tape and can subsequently be counted and analyzed using a Scanning Electron Microscope (SEM).

Another procedure, referred to as Wiped Particle Analysis (WPA), involves wiping a foam cloth material along the surface of the mounting arm 100 to remove any shed surface particles. The material is then analyzed to determine the amount of particle residue collected. A SEM can be used to count and analyze the shed surface particles on the material.

Liquid Particle Count (LPC) can be used. The LPC procedure involves dipping the mounting arm 100 in a fluid, such as deionized water. The immersed mounting arm 100 is subjected to ultrasonic waves that cause shed particles to be released from the surface of the mounting arm 100. The particles suspended in the fluid are counted and binned by sized using a laser sensor.

The repeatability of each procedure is difficult to know as the tests are destructive. However, averaged results examined at different heating temperatures illustrate a relative decrease in surface particle shedding of the mounting arm 100.

Referring to FIG. 4, both TPA and WPA results show a relative decrease in shed particle count with increasing temperature. FIG. 4 also provides projected WPA results, taking into account that only a portion of shed particles are actually accounted for in WPA.

Test data for WPA and LPC results are depicted in FIG. 5. The mounting arm 100, treated according to a method embodying the present invention, is compared to a control mounting arm. The control mounting arm is a typical mounting arm commonly sold on the market, and not subjected to treatment by the method embodied in the present invention. Comparing the control mounting arm results with those of the mounting arm 100 at 1500° F. and 1850° F., it is evident that the amount of particles on the surface is reduced substantially for an mounting arm 100 tested by the method of the present invention. FIG. 5 also provides test data for a mounting arm 100 fabricated using a method embodying the present invention and subjected to industry standard cleaning agents. The results depicted in FIG. 5 indicate that industry standard cleaning process actually increase the number of particles extracted from the surface rather than decrease it. This is attributed to the cleaning process's destructive effect on the pristine surface. Accordingly, the method embodying the present invention provides superior results in reducing surface particle shedding of the mounting arm 100.

While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that this invention not be limited to the specific constructions and arrangements shown and described, since various other changes, combinations, omissions, modifications and substitutions, in addition to those set forth in the above paragraphs, are possible. Those skilled in the art will appreciate that various adaptations and modifications of the just described preferred embodiment can be configured without departing from the scope and spirit of the invention. Therefore, it is to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein. 

1. A method for reducing surface shedding of a material formed from an alloy metal, the method comprising the steps of: heating an environment to a temperature sufficient to slightly soften the material; exposing the material to the heated environment for a predetermined time period; and allowing the material to cool to ambient temperature.
 2. The method of claim 1, wherein the predetermined time period is sufficient to soften the material's outer surface layer while substantially maintaining the material's structural characteristics.
 3. The method of claim 1, wherein the predetermined time period is sufficient to soften the material while substantially maintaining the material's structural characteristics.
 4. The method of claim 1, wherein the temperature of the heated environment is maintained above approximately 600° F.
 5. The method of claim 1, wherein the material undergoes a hardness change of at least 20 Vickers.
 6. The method of claim 1, wherein the heated environment is oxygen depleted.
 7. The method of claim 1, wherein the heated environment is oxygen depleted and filled with inert or reducing gas.
 8. The method of claim 1, wherein the heated environment is oxygen depleted and evacuated to a sub-atmospheric pressure sufficient to form a vacuum.
 9. The method of claim 1, wherein the alloy metal is a 300 Series stainless steel.
 10. A method for reducing surface shedding of a mounting arm formed from an alloy metal, the method comprising the steps of: heating an environment to a temperature sufficient to slightly soften the mounting arm; exposing the mounting arm to the heated environment for a predetermined time period; and allowing the mounting arm to cool to ambient temperature.
 11. The method of claim 10, wherein the predetermined time period is sufficient to soften the mounting arm's outer surface layer while substantially maintaining the mounting arm's structural characteristics.
 12. The method of claim 10, wherein the predetermined time period is sufficient to soften the mounting arm while substantially maintaining the mounting arm's structural characteristics.
 13. The method of claim 10, wherein the temperature of the heated environment is maintained above approximately 600° F.
 14. The method of claim 10, wherein the mounting arm undergoes a hardness change of at least 20 Vickers.
 15. The method of claim 10, wherein the heated environment is oxygen depleted.
 16. The method of claim 10, wherein the heated environment is oxygen depleted and filled with inert or reducing gas.
 17. The method of claim 10, wherein the heated environment is oxygen depleted and evacuated to a sub-atmospheric pressure sufficient to form a vacuum.
 18. The method of claim 10, wherein the alloy metal is a 300 Series stainless steel.
 19. A method for reducing surface particle shedding of a mounting arm formed from an alloy metal, the method comprising the steps of: heating an environment to a temperature sufficient to relieve residual stress in the mounting arm; exposing the mounting arm to the heated environment for a predetermined time period sufficient to relieve residual stress in the mounting arm's outer surface while substantially maintaining the mounting arm's yield strength; and allowing the mounting arm to cool to ambient temperature.
 20. The method of claim 19, wherein the temperature of the heated environment is maintained between approximately 600° F. and approximately 1850° F.
 21. The method of claim 19, wherein the environment is oxygen depleted.
 22. The method of claim 19, wherein the environment is oxygen depleted and filled with inert or reducing gas.
 23. The method of claim 19, wherein the environment is oxygen depleted and evacuated to a sub-atmospheric pressure sufficient to form a vacuum.
 24. The method of claim 19, wherein the mounting arm has a hardness of about 20 Vickers.
 25. The method of claim 19, wherein the alloy metal is a 300 Series stainless steel.
 26. A method for reducing surface scaling of a mounting arm formed from an alloy metal, the method comprising the steps of: providing an oxygen depleted environment; heating the environment to a temperature sufficient to relieve residual stress in the mounting arm while substantially maintaining the mounting arm's yield strength characteristics; exposing the mounting arm to the heated environment for a predetermined time period; and cooling the mounting arm to an ambient temperature.
 27. The method of claim 26, wherein the temperature of the heated environment is maintained between approximately 600° F. and approximately 1850° F.
 28. The method of claim 26, wherein the step of providing an oxygen depleted environment includes filling the environment with an inert or reducing gas.
 29. The method of claim 26, wherein the step of providing an oxygen depleted environment includes evacuating the environment to a sub-atmospheric pressure sufficient to form a vacuum.
 30. The method of claim 26, wherein the alloy metal is a 300 Series stainless steel.
 31. The method of claim 26, wherein the mounting arm has a hardness of about 20 Vickers. 